The present invention relates to an actuator for positioning heads in a magnetic disc apparatus, and more particularly to an actuator mechanism for positioning the heads, of the swing type that supports recording/reproducing magnetic heads such as to allow the swinging motion of them.
Typical examples of head positioning actuators of the type incorporated in a magnetic disc apparatus are disclosed, for example, in U.S. Pat. No. 4,391,035 and Japanese Utility Model Unexamined Publication No. 159566/1985. In these related disclosures, arms carrying magnetic heads are rotatably supported by a shaft, and a drive means including a coil is provided at the rear of the arms. Each of the arms is rotated about the shaft by electrically controlling the coil. Thus, the magnetic heads are positioned with respect to the surface of a magnetic disc.
In such known actuators, the coil which forms the magnetic circuit of a voice coil motor serving as the drive is incorporated in such a manner that the rear portion of the arms is resin-molded integrally with an arm base serving as a coil support member. This is advantageous in that the assembly of arms is simplified and in that there is no risk of the coil coming off due to vibration or the like.
Also, there is a demand for an improvement in the durability of the apparatus in a severe environment as well as a reduction in the size of a magnetic disc apparatus.
In the aforementioned prior devices, however, the arm base serving as the coil support member is resin-molded integrally with the rear portion of the arms and the coil. Thus, the thermal expansion coefficient of the coil differs from that of the resin. Accordingly, if such an apparatus is used when the temperature changes over a wide range, the arm base may be distorted, deformed or cracked due to the difference between the thermal expansion coefficients of the coil and the resin. This in that problem in that the reliability of the arm is low.
Also, in order to eliminate adverse influences due to external forces, it is necessary to balance a head arm carrying the heads with the arm base. However, in spite of the small size of the head arm, it is relatively heavy because it is made of a metal material. The arm base is provided with the coil, but its weight is reduced because the arm base is made of a molded resin. Therefore, in order to maintain the balance between the head arm and the head base, the volume of the arm base must be increased. This may hinder a reduction in the size of the apparatus.